LED components, such as housings, reflectors and reflector plates require an especially demanding combination of excellent color and improved physical properties. Ceramics may be advantageously used in those applications, but are still extremely costly and require demanding processing technologies.
Therefore, polymer compositions have been extensively studied and developed to replace ceramics as a lower cost material. LED applications require polymer compositions with good opacity and outstanding reflective properties. Various useful polymer compositions for LED applications are known, these ones usually include polycondensation polymers, such as polyphthalamides. One problem noted with the prior art compositions used in LED applications is that they tend to yellow when exposed to light and heat.
LED components are exposed to elevated temperatures during the manufacturing process. For example, during the fabricating steps the LED components are heated to about 180° C. to cure an epoxy or silicon encapsulant. The LED components are also exposed to temperatures above 260° C. while soldering operations are performed. In addition, while in use, LED components, such as automobile components, are routinely subjected to temperatures above 80° C. This exposure to high temperatures causes yellowing of polymer compositions used for forming LED components.
Desirably, reflector plates of LEDs and, in fine, the polymer compositions from which they are made should comply with a wide set of requirements, including, notably, high reflectance of light (in general, of visible light), high whiteness, good processability (e.g. good moldability), high dimensional stability (notably low coefficient of linear expansion), high mechanical strength, high heat deflection temperature and high heat resistance (low discoloration and low loss of reflectance when exposed to a high temperature).
Deterioration of components of reflectors may cause the LED devices to suffer from light distortion and/or poor emission efficiency after exposure to high temperature and high intensity radiation.
Titanium dioxide (TiO2) is extensively used in compositions used in LED applications as a white pigment. Such compositions exhibit superior properties for molding and the molded parts exhibit high thermal stability, including dimensional stability and retention of mechanical properties, during fabrication and end use. Retention of whiteness and reflectivity during processing and end use is also high. Typical compounds contain TiO2, a reinforcing filler (glass fiber or mineral fillers) and organic stabilizers (hindered phenols, phosphites and hindered amines light stabilizers).
WO 2011/027562 discloses LED reflector plates made of a composition comprising a polymer obtained by the polycondensation of 1,4-cyclohexane dicarboxylic acid and an aliphatic diamine comprising from 4-18 C atoms, optionally a white pigment such as TiO2, optionally glass fiber and/or wollastonite, optionally MgO or Mg(OH)2. The compositions prepared according to the examples all comprise 1.9% of MgO.
One of ordinary skill in the art will recognize that further improvements in heat stability, molding performance and reflectivity are advantageous for the development of LED assemblies. The utility of LED lighting devices of increasing power and brightness in electronics, in signage, in automobiles and in residential and commercial lighting has driven manufacturing and end use criteria to include even higher initial and retained reflectivity.
The inventors have found out that the presence of metal oxides such as magnesium oxide, zinc oxide and calcium oxide is surprisingly only advantageous when used in combination with TiO2 and with a polymer featuring a specific intrinsic melt viscosity at a specific moisture content.
The present invention provides a new solution to improved retention of reflectivity through the LED assembly manufacturing process (10 min at 260° C., 8 hours at 160° C.). The object of the invention is to provide a polyamide composition that offers high reflectivity in the molded part with high retention of whiteness and reflectivity after heat aging. The composition further provides improved retention of reflectivity after heat/light aging.